From time to time during the production of ethylene homopolymers and copolymers, including EPMs and EPDMs, it is necessary to shutdown the polymerization reactor for periodic maintenance and cleaning. During shutdown and start-up the inside of the reactor is exposed to air and moisture. Exposure of the internal surfaces of the carbon steel reactor to air and moisture can result in surface corrosion due to one or more of the catalyst components (precursor transition metal compound, co-catalyst, and/or promoter) reacting with the air or moisture. Exposed surfaces of other carbon steel equipment used along with the reactor such as recycle lines and elbows, compressor impeller, recycle valve, distribution caps, and coupons can likewise be affected.
Exposure to air and moisture is particularly a problem during the production of EPMs and EPDMs in which an alkylaluminum or alkylaluminum halide is employed as co-catalyst in conjunction with a vanadium catalyst and a halocarbon promoter. During polymerization, the alkylaluminum or alkylaluminum halide is chlorinated as the result of reacting with the halocarbon promoter. This results in the formation of aluminum compounds with a Cl/Al ratio greater than one. These chlorinated and polychlorinated species are a source of reactor corrosion when the reactor is exposed to air and moisture. Upon exposure to air and moisture the chlorinated aluminum species immediately hydrolyze to produce inert alumina and hydrochloric acid. Hydrochloric acid in the presence of air or moisture corrodes the metal surfaces of the reactor.
The industry has attempted to solve the problem of corrosion of the internal reactor surfaces by painting or coating them with polymers or anti-corrosion paints to extend the lifetime of the reactor. Painting is costly and time consuming and chips of the paint or coating can form flakes which can fall into the reactor during polymerization. Accordingly, there is a need for another method of eliminating or reducing corrosion of internal reactor surfaces.